Creating a Dockerfile

A Dockerfile is a text file with instructions to build a custom image. It automates the process of creating an image so you can reproduce it anywhere.

What Is a Dockerfile?

A Dockerfile contains a series of commands that Docker executes to assemble an image. Each command adds a new layer to the image. By version‑controlling your Dockerfile, you keep your infrastructure as code.

Basic Dockerfile Instructions

FROM: Sets the base image (e.g., FROM ubuntu:22.04). Every Dockerfile must start with FROM.
WORKDIR: Sets the working directory for subsequent instructions.
COPY: Copies files from your host into the image.
RUN: Executes a command during the build (e.g., install packages).
CMD: Specifies the default command to run when a container starts. There can be only one CMD.
EXPOSE: Documents which port the container listens on (does not publish it).

Example: Simple Python App

Create a file named app.py with:

print("Hello from Docker")

Now create a Dockerfile (no extension) in the same folder:

FROM python:3.11-slim
WORKDIR /app
COPY app.py .
CMD ["python", "app.py"]

Understanding Each Line

FROM python:3.11-slim – starts from a small Python image.
WORKDIR /app – creates the /app directory and switches to it.
COPY app.py . – copies app.py from your host into /app inside the image.
CMD ["python", "app.py"] – the default command when the container runs.

Two Minute Drill

Dockerfile automates image creation.
Key instructions: FROM, WORKDIR, COPY, RUN, CMD.
Each instruction creates a new image layer.
Store your Dockerfile in version control for reproducibility.

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Creating a Dockerfile

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